Color imager bar based spectrophotometer for color printer color control system

ABSTRACT

An improved and lower cost color spectrophotometer, especially suitable for on-line color printer color control systems, incorporating a low cost commercial imaging chip, which normally only forms part of a three row, three color, document imaging bar used for imaging documents in scanners, digital copiers, or multifunction products, having multiple photo-sites with at least three different color filters in three rows. This multiple photo-sites chip may be modified to also provide unfiltered photo-sites. This spectrophotometer may have a substantially reduced number of different LED or other spectral illumination sources, one of which may be for white light, yet provide multiple spectral data outputs from the differently filtered photo-sites being simultaneous illuminated by the light reflected from a color test target area which is being sequentially illuminated by the respective limited number of LEDs, enabling broad spectrum information and color control.

[0001] Cross-reference and incorporation by reference is made to thefollowing copending and commonly assigned U.S. patent applications: U.S.application Ser. No. 09/448,987, filed Nov. 24, 1999, Attorney DocketNo. D/99511 Q, and U.S. application Ser. No. 09/449,263, filed Nov. 24,1999, Attorney Docket No. D/99511Q1, both by the same Lingappa K.Mestha; and U.S. application Ser. No. 09/535,007, filed Mar. 23, 2000,by Fred F. Hubble, III and Joel A. Kubby, Attorney Docket No. D/99511I;and U.S. application Ser. No.______ , filed ______ 2001, by Fred F.Hubble, III, Tonya A. Love and Daniel A. Robins, Attorney Docket No.D/A1024 entitled “Angular, Azimuthal and Displacement InsensitiveSpectrophotometer For Color Printer Color Control Systems.”

[0002] Disclosed in the embodiments herein is an improved, low cost,plural color spectrophotometer for color calibration or correctionsystems, highly suitable to be used for, or incorporated into, the colorcalibration or control of various color printing systems or otheron-line color control or color processing systems. The exemplarydisclosed spectrophotometer desirably utilizes (incorporates in part) alow cost component or part of a low cost commercially available multiplephoto-sites, plural spectral responsive, imaging array or bar, such asheretofore used for imaging colored documents in various scanners,digital copiers, and multifunction products. Also disclosed is arelatively simple modification thereof to provide additional differentlyspectral responsive photo-sites.

[0003] Also disclosed herein is a low cost spectrophotometer which mayemploy a small limited number of different spectra LED or otherillumination sources, yet providing multiple data outputs from a lowcost photosensor having plural different spectral responsive photo-sitesdetecting light reflected by a colored test target area sequentiallyilluminated by those illumination sources (or continuously white lightilluminated), to rapidly provide broad spectrum data from a colored testsurface.

[0004] By way of background, examples of full color document imagingbars include those used in various document scanning systems of variouswell known Xerox Corporation commercial products (including some beingalternatively used for black and white imaging) such as the DocumentCenter 255DC™ products, or the Document Center Color Series 50™products. Some examples of patents relating to semiconductor colorimager bars or segments thereof and their operation or circuitry includeXerox Corporation U.S. Pat. No. 5,808,297, issued Sep. 15, 1998; U.S.Pat. No. 5,543,838, issued Aug. 6, 1996; U.S. Pat. No. 5,550,653, issuedAug. 27, 1996; U.S. Pat. No. 5,604,362, issued Feb. 18, 1997; and U.S.Pat. No. 5,519,514, issued May 21, 1996. Typically, such color imagingbars come already provided with at least three different color filters,such as red, green and blue, overlying three rows of closely spacedlight sensor elements (photo-sites), to provide electrical outputsignals corresponding to the colors of the document image being scanned.Such imaging bars are typically formed by edge butting together a numberof individual imaging chips, each having such multiple tiny and closelyspaced photo-sites. Typically, there are three rows of such photo-siteson each such chip, as in the assembled imaging bar, with said integralfilters for red, green and blue, respectively.

[0005] Because of the high volumes in which such commercial colorimaging bars are made for such products, it has been discovered thattheir manufacturers can provide, at low cost, a commercial source ofsaid single imaging chip components thereof. The fact that each suchchip can provide electrical signals from multiple light sensor elements(photo-sites) in at least three rows of different spectral responseswhich are closely enough spaced together so as to be simultaneouslyilluminated by a relatively small area of illumination, is effectivelyutilized in the spectrophotometer of the embodiments herein. (It will beunderstood that the term “chip” as used herein does not exclude the useof two or more such chips, either integrally abutted or separatelypositioned.)

[0006] However, it is not believed that heretofore such plural sensorschips for plural color sensing, which are normally put together inseries for imaging bars for document scanning, have ever been used inspectrophotometers. These chips themselves are not normally even soldindividually. The disclosed embodiment illustrates how that may be done,to provide a compact and lower cost spectrophotometer especiallysuitable for on-line color control systems for sensing the colors ofmoving printed sheets or other color materials.

[0007] Although not limited thereto, the exemplary spectrophotometer ofthe embodiment herein is shown and described herein in desirablecombination as an integral part of an automatic on-line continuous colortable correction system of a color printer, in which this low costspectrophotometer may be affordably provided in the output path of eachcolor printer for automatic measurement of printed color test patches ofprinter output, without any manual effort or intervention beingrequired. Such color control systems are further described in the aboveand below cited co-pending applications and patents. For example, inXerox Corp. U.S. Pat. No. 6,178,007 B1, issued Jan. 23, 2001, based onU.S. application Ser. No. 08/786,010, filed Jan. 21, 1997 by Steven J.Harrington, Attorney Docket No. D/96644, entitled “Method For ContinuousIncremental Color Calibration For Color Document Output Terminals.” TheEuropean patent application equivalent thereof was published by theEuropean Patent Office on Jul. 22, 1998 as EPO Publication No. 0 854 638A2. Also, Xerox Corp. U.S. Pat. No. 6,222,648, issued Apr. 24, 2001,based on U.S. application Ser. No. 08/787,524, also filed Jan. 21, 1997,by Barry Wolf, et al, entitled “On Line Compensation for Slow Drift ofColor Fidelity in Document Output Terminals (DOT)”, Attorney Docket No.D/96459. Also noted in this regard are Xerox Corp. U.S. Pat. No.6,157,469, issued Dec. 5, 2000 and filed May 22, 1998 by Lingappa K.Mestha; Apple Computer, Inc. U.S. Pat. No. 5,881,209, issued 1999; U.S.Pat. No. 5,612,902 issued Mar. 18, 1997 to Michael Stokes, and otherpatents and applications further noted below.

[0008] A low cost, relatively simple, spectrophotometer, as disclosedherein, is thus particularly (but not exclusively) highly desirable forsuch a “colorimetry” function for such an on-line printer colorcorrection system. Where at least one dedicated spectrophotometer isprovided in each printer, its cost and other factors becomes much moresignificant, as compared to the high cost (and other unsuitability's foron-line use) of typical laboratory spectrophotometers.

[0009] An early patent of interest as to using a colorimeter in theprinted sheets output of a color printer is Xerox Corp. U.S. Pat. No.5,748,221, issued May 5, 1998 to Vittorio Castelli, et al, filed Nov. 1,1995 (D/95398). This patent is also of particular interest here for itsCol. 6, lines 18 to 28 description of measuring color:

[0010] “. . . by imaging a part of an illuminated color patch on threeamorphous silicon detector elements after filtering with red, green andblue materials. The technology is akin to that of color input scanners.The detector outputs can be used as densitometric values to assure colorconsistency. Calibration of the resulting instrument outputs againstmeasurement by laboratory colorimeters taken over a large sample ofpatches made by the toners of the printer of interest allows mapping toabsolute color coordinates (such as L*a*b*).”

[0011] As disclosed in above-cited references, automatic on-line colorrecalibration systems can be much more effective with an on-line colormeasurement system where a spectrophotometer may be mounted in the paperpath of the moving copy sheets in the printer, preferably in the outputpath after fusing or drying, without having to otherwise modify theprinter, or interfere with or interrupt normal printing, or the movementof the printed sheets in said paper path, and yet provide accurate colormeasurements of test color patches printed on the moving sheets as theypass the spectrophotometer. That enables a complete closed loop colorcontrol of a printer.

[0012] However, it should be noted that color measurements, and/or theuse of color measurements for various quality or consistency controlfunctions, are also important for many other different technologies andapplications, such as in the production of textiles, wallpaper,plastics, paint, inks, etc. Thus, the disclosed color detection systemmay have applications in various such other fields where these materialsor objects are to be color tested. Although the specific exemplaryembodiment herein is part of a preferred automatic recalibration systemwith an on-line color printer color spectrophotometer, it will beappreciated that the disclosed spectrophotometer is not limited to thatdisclosed application.

[0013] By way of general background, studies have demonstrated thathumans are particularly sensitive to spatial color variations. Typicalfull color printing controls, as well as typical color controls in othercommercial industries, still typically utilize manual off-line colortesting and frequent manual color adjustments by skilled operators. Boththe cost and the difficulty of on-line use of prior color measurementapparatus and control systems, and the need for manual recalibrationsteps, has heretofore inhibited automation of many of such variouscommercial color testing and color adjustment systems. The disclosedlower cost spectrophotometer addresses both of those concerns.

[0014] By way of some examples of the construction or design of variousother color spectrophotometers themselves, besides Xerox Corp. U.S. Pat.No. 5,748,221 above, and, especially, the above cross-referenced U.S.application Ser. No. 09/535,007, filed Mar. 23, 2000 by Fred F. Hubble,III and Joel A. Kubby, there is noted HP U.S. Pat. No. 5,671,059, issued1993; and HP U.S. Pat. No. 5,272,518, issued Dec. 21, 1993; AccuracyMicrosensor, Inc. U.S. Pat. No. 5,838,451 and U.S. Pat. No. 5,137,364,both issued to Cornelius J. McCarthy on Nov. 17, 1998 and Aug. 11, 1992,respectively; Color Savvy U.S. Pat. Nos. 6,147,761, 6,020,583,5,963,333; BYK-Gardner U.S. Pat. No. 5,844,680; and Colorimeter U.S.Pat. No. 6,157,454.

[0015] Also of background interest here is that white (instead of narrowspectrum) LED illuminators and plural sensors with different colorfilters are disclosed in an EP Patent application Ser. No. 0 921 381 A2,published Sep. 6, 1999 for a color sensor for inspecting color print onnewspaper or other printed products.

[0016] By way of further background, or expressing it in other words,for a desirably low cost implementation of a spectrophotometer withplural light emitting diodes (LEDs) as the respective different colorlight sources, LEDs of different colors may be selected and switched onindividually in sequence to illuminate a test target for a brief lengthof time sufficient for enough information to be extracted by a photocellof the reflectance spectra of the substrate. Over a number of years, aconcentrated effort in the Xerox Corporation Wilson Research Center hasdesigned and built a relatively low cost experimental spectrophotometerusing, for example, 10 LEDs, as part of a printer color control systemdynamically measuring the color of test patches on the printed outputmedia “on line”, that is, while the media is still in the sheettransport or paper path of a print engine, for real-time and fullyautomatic printer color correction applications. A limited example ofthat color control system capability was presented in a restrictedpublic technology capability demonstration by Xerox Corporation at theinternational “Drupa 2000” show in Germany (without public disclosure ofthe hardware, software or technical details, or any offers to sell).Further details of the specific spectrophotometer embodiment so utilizedare disclosed in the prior above first-paragraph cross-referenced patentapplication by Fred F. Hubble, III and Joel A. Kubby. Each LED thereofwas selected to have a narrow band response curve in the spectral space.Ten LEDs provided 10 color calibration measurements on the spectralreflectance curve. The LEDs are switched on one at a time and thereflected light was detected by a single photodetector as aphoto-current which may be integrated for few milliseconds to give avoltage output. Thus, 10 voltage outputs per each measured color testpatch are available with such a spectrophotometer using 10 LEDs. Thesevoltages may be converted directly to L*a*b* color space, or to 10reflectance values and then to L*a*b* color space coordinates (ifneeded). The cost of that LED spectrophotometer hardware includes thehead for mounting the 10 spaced LEDs, the lenses, and the basicswitching electronics.

[0017] Other than the above Xerox Corp. experimental spectrophotometers,some others presently known include a grating-based spectrophotometermade by Ocean Optics Inc., LED based sensors marketed by “ColorSavvy” orAccuracy Microsensor (such as in their above-cited patents); and otherspectrophotometers by Gretag MacBeth (Viptronic), ExColor, and X-Rite(DTP41). However, those other spectrophotometers are believed to havesignificant cost, measurement time, target displacement errors, and/orother difficulties, for use in real-time printer on-line measurements.

[0018] For maintaining or lowering the UMC (unit manufacturing cost) ofcolor printers in which a dedicated on-line spectrophotometer and itscircuitry would need to be provided in each printer, there is a furtherneed to further bring down the cost of a suitably fast, yet suitablywide spectral range, spectrophotometer. If the spectrophotometer costcan be sufficiently reduced, it may be practicable as well as desirableto provide an on-line output color control system for many or mostfuture color printers, even relatively low cost color printers. That isbecause, as taught in art cited herein and elsewhere, other componentsand features of such an on-line printer color control system can belargely implemented in software, which has little incremental UMC, byimplementing color correction tables, steps and/or algorithms insoftware and digital memory. (See, for example, the above-cited XeroxCorp. Steven J. Harrington U.S. Pat. No. 6,178,007 B1, and other artcited therein and/or above, including Xerox Corp. U.S. Pat. No.6,157,469.)

[0019] It is believed that a spectrophotometer of the novel typedisclosed herein, utilizing a component chip or portion of a low UMCcommercially available color image sensor array or bar, such as imagerbars mass produced for commercial use in document scanners, combinedwith suitable LEDs or other light sources so as to provide aspectrophotometer of suitable speed and spectral outputs, has thepotential to give even greater speed at even lower cost than theabove-described prior low cost 10 LED Xerox Corp. LED spectrophotometer.

[0020] As used in the patent claims and elsewhere herein, unlessotherwise specifically indicated, the term “spectrophotometer” mayencompass a spectrophotometer, calorimeter, and densitometer, as broadlydefined herein. That is, the word “spectrophotometer” may be given thebroadest possible definition and coverage in the claims herein,consistent with the rest of the claim. The definition or use of suchabove terms may vary or differ among various scientists and engineers.However, the following is an attempt to provide some simplifiedclarifications relating and distinguishing the respective terms“spectrophotometer,” “colorimeter,” and “densitometer,” as they may beused in the specific context of specification examples of providingcomponents for an on-line color printer color correction system, but notnecessarily as claim limitations.

[0021] A typical “spectrophotometer” measures the reflectance of anilluminated object of interest over many light wavelengths. Typicalprior spectrophotometers in this context use 16 or 32 channels measuringfrom 400 nm to 700 nm or so, to cover the humanly visible color spectraor wavelength range. A typical spectrophotometer gives color informationin terms of measured reflectances or transmittances of light, at thedifferent wavelengths of light, from the test surface. (This is tomeasure more closely to what the human eye would see as a combined imageof a broad white light spectra image reflectance, but thespectrophotometer desirably provides distinct electrical signalscorresponding to the different levels of reflected light from therespective different illumination wavelength ranges or channels.)

[0022] A “colorimeter” normally has three illumination channels, red,green and blue. That is, generally, a “colorimeter” provides its three(red, green and blue or “RGB”) values as read by a light sensor ordetector receiving reflected light from a color test surfacesequentially illuminated with red, green and blue illuminators, such asthree different color LEDs or three lamps with three different colorfilters. It may thus be considered different from, or a limited specialcase of, a “spectrophotometer,” in that it provides output colorinformation in the trichromatic quantity known as RGB.

[0023] Trichromatic quantities may be used for representing color inthree coordinate space through some type of transformation. Other RGBconversions to “device independent color space” (i.e., RGB converted toconventional L*a*b*) typically use a color conversion transformationequation or a “lookup table” system in a known manner. (Examples areprovided in references cited herein, and elsewhere.)

[0024] A “densitometer” typically has only a single channel, and simplymeasures the amplitude of light reflectivity from the test surface, suchas a developed toner test patch on a photoreceptor, at a selected angleover a range of wavelengths, which may be wide or narrow. A singleillumination source, such as an IR LED, a visible LED, or anincandescent lamp, may be used. The output of the densitometer detectoris programmed to give the optical density of the sample. A densitometerof this type is basically “color blind.” For example, a cyan test patchand magenta test patch could have the same optical densities as seen bythe densitometer, but, of course, exhibit different colors.

[0025] A multiple LED reflectance spectrophotometer, as in the examplesof the embodiments herein, may be considered to belong to a special caseof spectrophotometers which normally illuminate the target with narrowband or monochromatic light. Others, with wide band illuminationsources, can be flashed Xenon lamp spectrophotometers, or incandescentlamp spectrophotometers. A spectrophotometer is normally programmed togive more detailed reflectance values by using more than 3 channelmeasurements (for example, 10 or more channel measurements), withconversion algorithms. That is in contrast to normal three channelcalorimeters, which cannot give accurate, human eye related, reflectancespectra measurements, because they have insufficient measurements forthat (only 3 measurements).

[0026] The spectrophotometer of the disclosed embodiment is aspectrophotometer especially suitable for being mounted at one side ofthe printed sheets output path of a color printer to optically evaluatecolor imprinted output sheets as they move past the spectrophotometer,variably spaced therefrom, without having to contact the sheets orinterfere with the normal movement of the sheets. In particular, it maybe used to measure a limited number of color test patch samples printedby the printer on actual printed sheet output of the printer duringregular or selected printer operation intervals (between normal printingruns or print jobs). These color test sheet printing intervals may be atregular timed intervals, and/or at each machine “cycle-up,” or asotherwise directed by the system software. The spectrophotometer may bemounted at one side of the paper path of the machine, or, if it isdesired to use duplex color test sheets, two spectrophotometers may bemounted on opposite sides of the paper path.

[0027] Relatively frequent color recalibration of a color printer ishighly desirable, since the colors actually printed on the output media(as compared to the colors intended to be printed) can significantlychange, or drift out of calibration over time, for various knownreasons. For example, changes in the selected or loaded print media,such as differences paper or plastic sheet types, materials, weights,calendaring, coating, humidity, etc. Or changes in the printer's ambientconditions, changes in the image developer materials, aging or wear ofprinter components, varying interactions of different colors beingprinted, etc. Printing test color patches on test sheets of the sameprint media under the same printing conditions during the same relativetime periods as the color print job being color-controlled is thus verydesirable.

[0028] It is thus also advantageous to provide dual-mode color testsheets, in which multiple color patches of different colors are printedon otherwise blank areas of each, or selected, banner, cover, or otherinter-document or print job separator sheets. Different sets of colorsmay be printed on different banner or other test sheets. This dual useof such sheets saves both print paper and printer utilization time, andalso provides frequent color recalibration opportunities where theprinting system is one in which banner sheets are being printed atfrequent intervals anyway.

[0029] An additional feature which can be provided is to tailor or setthe particular colors or combinations of the test patches on aparticular banner or other test sheet to those colors which are about tobe printed on the specific document for that banner sheet, i.e., thedocument pages which are to be printed immediately subsequent to thatbanner sheet (the print job identified by that banner sheet). This canprovide a “real time” color correction for the color printer which istailored to correct printing of the colors of the very next document tobe printed.

[0030] The preferred implementations of the systems and featuresdisclosed herein may vary depending on the situation. Also, various ofthe disclosed features or components may be alternatively used for suchfunctions as gray scale balancing, turning on more than one illuminationsource at once, such as oppositely positioned LEDs, etc.

[0031] It will be appreciated that these test patch images and colorsmay be automatically sent to the printer imager from a stored data filespecifically designed for printing the dual mode banner sheet or othercolor test sheet page, and/or they may be embedded inside the customerjob containing the banner page. That is, the latter may be directlyelectronically associated with the electronic document to be printed,and/or generated or transmitted by the document author or sender.Because the printed test sheet color patches colors and their printingsequence is known (and stored) information, the on-linespectrophotometer measurement data therefrom can be automaticallycoordinated and compared.

[0032] After the spectrophotometer or other color sensor reads thecolors of the test patches, the measured color signals may beautomatically processed inside the system controller or the printercontroller to produce or modify the tone reproduction curve, asexplained in the cited references. The color test patches on the nexttest sheet may then be printed with that new tone reproduction curve.This process may be repeated so as to generate further corrected tonereproduction curves. If the printer's color image printing componentsand materials are relatively stable, with only relatively slow long termdrift, and there is not a print media or other abrupt change, the tonereproduction curve produced using this closed loop control system willbe the correct curve for achieving consistent colors for at least one oreven a substantial number of customer print jobs printed thereafter, andonly relatively infrequent and few color test sheets, such as the normalbanner sheets, need be printed.

[0033] However, if there are substantial changes in the print mediabeing used by the printer, or other sudden and major disturbances in theprinted colors (which can be detected by the spectrophotometer output inresponse to the test patches on the next dual mode banner sheet or othercolor test sheet or even, in certain instances, in the imprinted images)then the subsequent customer print job may have incorrect colorreproduction. In these situations of customer print media changes in theprinter (or new print jobs or job tickets that specify a change in printmedia for that print job), where that print media change is such that itmay substantially affect the accuracy of the printed colors for thatsubsequent print job, it is not desirable to continue printing and thenhave to discard the next subsequent print jobs printed with customerunacceptable colors. In that situation it may be preferable in colorcritical applications to interrupt the normal printing sequence once thesudden color printing disturbance is detected and to instead printplural additional color test sheets in immediate succession, withdifferent color test patch colors, to sense and converge on a new tonereproduction curve that will achieve consistent color printing for thatnew print media, and only then to resume the normal printing sequence ofcustomer print jobs. Thus, the subsequent customer print jobs would thenuse the final, re-stabilized, tone reproduction curve obtained aftersuch a predetermined number of sequential plural color test sheets havebeen printed.

[0034] This patent application is not related to or limited to anyparticular one of the various possible (see, for example, various of thecited references) algorithms or mathematical techniques for processingthe electronic signals from the spectrophotometer to generate or updatecolor correction tables, tone reproduction curves, or other colorcontrols, and hence they need not be further discussed herein.

[0035] Various possible color correction systems can employ the outputsignals of spectrophotometers, using various sophisticated feedback,correction and calibration systems, which need not be discussed in anyfurther detail here, since the general concepts and many specificembodiments are disclosed in many other patents (including those citedherein) and publications. In particular, to electronically analyze andutilize the spectrophotometer or other electronic printed color outputinformation with a feedback analysis system for the color controlsystems for a printer or other color reproduction system. It is,however, desirable in such systems to be able to use a substantiallyreduced (smaller) number of color patch samples, printed at intervalsduring the regular printing operations, to provide relativelysubstantially continuous updating correction of the printer's colorrenditions over a wide or substantially complete color spectra. Notedespecially in that regard is the above-cited Xerox Corp. Steven J.Harrington U.S. Pat. No. 6,178,007 B1.

[0036] Color correction and/or color control systems should not beconfused with color registration systems or sensors. Those systems arefor insuring that colors are correctly printed accurately superposedand/or accurately adjacent to one another, such as by providingpositional information for shifting the position of respective colorimages being printed.

[0037] Other background patents which have been cited as to colorcontrol or correction systems for printers include the following U.S.patents: Xerox Corp. U.S. Pat. No. 5,963,244, issued Oct. 5, 1999 to L.K. Mestha, et al, entitled “Optimal Reconstruction of Tone ReproductionCurve” (using a lookup table and densitometer readings of photoreceptorsample color test patches to control various color printer parameters);U.S. Pat. No. 5,581,376, issued December 1996 to Harrington; U.S. Pat.No. 5,528,386, issued Jun. 18, 1996 to Rolleston et al.; U.S. Pat. No.4,275,413, issued Jun. 23, 1981 to Sakamoto et al.; U.S. Pat. No.4,500,919, issued Feb. 19, 1985 to Schreiber; U.S. Pat. No. 5,416,613,issued May 16, 1995 to Rolleston et al.; U.S. Pat. No. 5,508,826, filedApr. 27, 1993 and issued Apr. 16, 1996 to William J. Lloyd et al.; U.S.Pat. No. 5,471,324, issued Nov. 28, 1995 to Rolleston; U.S. Pat. No.5,491,568, issued Feb. 13, 1996 to Wan; U.S. Pat. No. 5,539,522, issuedJul. 23, 1996 to Yoshida; U.S. Pat. No. 5,483,360, issued Jan. 9, 1996to Rolleston et al.; U.S. Pat. No. 5,594,557, issued January 1997 toRolleston et al.; U.S. Pat. No. 2,790,844 issued April 1957 toNeugebauer; U.S. Pat. No. 4,500,919, issued February 1985 to Schreiber;U.S. Pat. No. 5,491,568, issued Feb. 13, 1996 to Wan; U.S. Pat. No.5,481,380 to Bestmann, issued Jan. 2, 1996; U.S. Pat. No. 5,664,072,issued Sep. 2, 1997 to Ueda et al.; U.S. Pat. No. 5,544,258, issued Aug.6, 1996 to Levien; and U.S. Pat. No. 5,881,209, filed Sep. 13, 1994 andissued Mar. 9, 1999 to Michael Stokes.

[0038] By way of further background on the subject of technology forautomatic color correction for color printers or other reproductionapparatus, especially such systems utilizing feedback signals from acalorimeter or spectrophotometer (as noted, those terms may be usedinterchangeably herein), and/or automatically measuring the actuallyprinted colors of test patches on printed copy sheets as they are beingfed through the output path the printer, there is noted the following:the above-cited Xerox Corp. U.S. Pat. No. 5,748,221, filed Nov. 1, 1995and issued May 5, 1998 to V. Castelli, et al, entitled “Apparatus forColorimetry, Gloss and Registration Feedback in a Color PrintingMachine,” (noting especially the calorimeter detector details); theabove-cited Apple Computer, Inc. U.S. Pat. No. 5,612,902, issued Mar.18, 1997 to Michael Stokes; Xerox Corp. U.S. Pat. No. 5,510,896, issuedApr. 23, 1996 to Walter Wafler, filed Jun. 18, 1993 (see especially Col.8 re color calibration from information from a scanned color test copysheet as compared to original color image information); and Xerox Corp.U.S. Pat. No. 5,884,118, issued Mar. 16,1999 to Mantell and L. K.Mestha, et al, entitled “Printer Having Print Output Linked to ScannerInput for Automated Image Quality Adjustment” (note especially Col. 6,lines 45-49).

[0039] U.S. Patents of interest to color correction in general, butwhich may be useful with, or provide background information for, theabove or other systems, include the above-cited Xerox Corp. U.S. Pat.No. 5,594,557, filed Oct. 3, 1994 and issued Jan. 14, 1997 to R. J.Rolleston et al., entitled “Color Printer Calibration Correcting forLocal Printer Non-Linearities,” Seiko Epson Corp. U.S. Pat. No.5,809,213, provisionally filed Feb. 23, 1996 and issued Sep. 15, 1998 toA. K. Bhattacharjya re reduced color measurement samples; and SplashTechnology, Inc. U.S. Pat. No. 5,760,913, filed Feb. 12, 1996 and issuedJun. 2, 1998 to Richard A. Falk in which a calibration image is scannedusing a scanner coupled to the printing system with a personal computer.

[0040] In addition to above-cited issued patents, also noted as ofpossible interest to on-line color printer color control or correctionsystems (other than spectrophotometers per se) are Xerox Corp. U.S.Applications including: U.S. application Ser. No. 09/083,202, filed May22, 1998 by Mark A. Scheuer, et al., entitled “Device Independent ColorController and Method,” Attorney Docket No. D/97695; U.S. applicationSer. No. 09/083,203, filed May 22, 1998 by Lingappa K. Mestha, entitled“Dynamic Device Independent Image,” Attorney Docket No. D/98203 (nowU.S. Pat. No. 6,157,469, issued Dec. 5, 2000); U.S. application Ser. No.09/232,465, filed Jan. 19, 1999 by Martin E. Banton, et al., entitled“Apparatus and Method for Using Feedback and Feedforward in theGeneration of Presentation Images In A Distributed Digital ImageProcessing System,” Attorney Docket No. D/98423; U.S. application Ser.No. 09/221,996, filed Dec. 29, 1998 by Lingappa K. Mestha, et al.,entitled “Color Adjustment Apparatus and Method,” Attorney Docket No.D/98428; U.S. application Ser. No. 09/455,761, filed Dec. 7, 1999 bySidney W. Marshall, et al., entitled “Color Gamut Mapping for AccuratelyMapping Certain Critical Colors and Corresponding Transforming of NearbyColors and Enhancing Global Smoothness,” Attorney Docket No. D/99087;U.S. application Ser. No. 09/487,586, filed Jan. 19, 2000 by Lingappa K.Mestha, et al., entitled “Methods For Producing Device and IlluminationIndependent Color Reproduction,” Attorney Docket No. D/99159; U.S.application Ser. No. 09/451,215, filed Nov. 29, 1999 by Lingappa K.Mestha, et al., entitled “On-Line Model Prediction and CalibrationSystem For A Dynamically Varying Color Marking Device,” Attorney DocketNo. D/99508; U.S. application Ser. No. 09/454,431, filed Dec. 3, 1999 byTracy E. Thieret, et al., entitled “On-Line Piecewise Homemorphism ModelPrediction, Control and Calibration System for a Dynamically VaryingColor Marking Device,” Attorney Docket No. D/99577Q; U.S. applicationSer. No. 09/461,072, filed Dec. 15, 1999 by Lingappa K. Mestha, et al.,entitled “Systems and Methods for Device Independent Color Control toAchieve Accurate Color Proofing and Reproduction,” Attorney Docket No.D/99627; U.S. application Ser. No. 09/562,072, filed May 1, 2000 byLingappa K. Mestha, et al., entitled “System and Method forReconstruction of Spectral Curves, Using Measurements from a ColorSensor and Statistical Techniques,” Attorney Docket No. D/99803; U.S.application Ser. No. 09/621,860, filed Jul. 21, 2000 by Lingappa K.Mestha, et al., entitled “System and Method for Reconstruction ofSpectral Curves Using Measurements from a Color Sensor and a SpectralMeasurement System Model,” Attorney Docket No. D/A0098; and U.S.application Ser. No. 09/566,291, filed May 5, 2000 by Lingappa K.Mestha, et al., entitled “On-Line Calibration System For A DynamicallyVarying Color Marking Device,” Attorney Docket No. D/A0102.

[0041] As further well-known background for on difficulties in colorcorrection of printers in general, computers and other electronicequipment generating and inputting color images or documents typicallygenerate three-dimensional or RGB (red, green, blue) color signals.These color signals may be transmitted as PDL or other deviceindependent terms to a specific server or printer for a “RIP” (rasterimage process) conversion to device dependent color values, such as forthe line and bit signals for the laser scanner or LED bar of theparticular printer. Many printers, however, can receive four-dimensionalor CMYK (cyan, magenta, yellow, and black) signals as input, and/or canprint with four such print colors (although the printed images can stillbe measured as corresponding RGB values). A look-up table is commonlyprovided to convert each digital RGB color signal value to acorresponding digital CMYK value before or after being received by theprinter.

[0042] Real-world printers inherently have non-ideal printing materials,colors and behaviors, and therefore have complex non-linear colorimetricresponses. Also, interactions between the cyan, magenta, and yellowimaging materials exist, especially on the printed output, which resultin unwanted or unintended absorptions and/or reflections of colors. Evenafter a printer is initially calibrated, such that one or a range ofinput digital CMYK values produce proper colors, the full spectrum ofCMYK values and printed colors will not be or remain fully accurate. Inother words, the colors requested or directed to be printed by variousinput signals will not be the same as the actual colors printed.

[0043] This discrepancy arises in part because the relationship betweenthe digital input values that drive the printer and the resultingcalorimetric response is a complex non-linear function. Labeling theresponse, or other values, as “colorimetric” can indicate that theresponse or value has been measured by such an instrument. Adequatelymodeling the colorimetric response of a printer to achieve linearityacross the entire available spectrum requires many parameters.Typically, a color correction look-up table is built which approximatesthe mapping between RGB colorimetric space and CMYK values, as taught invarious of the above-cited references. Each RGB coordinate may betypically represented by an 8-bit red value, an 8-bit green value, andan 8-it blue value. Although those RGB coordinates are capable ofaddressing a look-up table having 256³ locations, measuring and storing256³ values is time consuming and expensive. The look-up table is thustypically partitioned into a smaller size such as 16×16×16 (4096) tablelocations, each of which stores a four-dimensional CMYK value. OtherCMYK values may then be found by interpolating the known CMYK valuesusing an interpolation process, for example, trilinear or tetrahedralinterpolation.

[0044] The color correction look-up table may be built by sending a setof CMYK digital values to the printer, measuring the calorimetric RGBvalues of the resulting color patches outputted by the printer with aspectrophotometer, and generating the look-up table from the differencebetween the inputted values and the measured outputted values. Morespecifically, the color correction look-up table corrects fornon-linearities, printing parameter variations, and unwanted absorptionsof inks, so that the printer will print the true corresponding color.

[0045] After the color correction table is generated, the actual printerresponse may tend to drift over time. To correct for the drift, thesystem is adjusted or recalibrated periodically. Recalibrating the colorcorrection table involves periodically printing and remeasuring a set oftest color patches which are then compared to an original set of colorpatches by calibration software. Remeasuring, however, has heretoforemore typically been performed manually by a scanner or other measuringdevice which is remote from the printer being recalibrated. For example,by removing a test output sheet from the printer output tray, placing it(stationary) on a table and sliding a spectrophotometer over it,manually or with an X-Y plotter driver, or automatically feeding thetest sheet through the spectrophotometer, and storing thespectrophotometer output signals data in an associated memory to readout later, or connecting the spectrophotometer by an electrical wire orcable to the printer controller or its server to directly receive thosecolor recalibration electrical input signals from the spectrophotometerand process them as described. The connecting cable could be replaced byknown IR or RF wireless (such as “BlueTooth”) connection systems, asused in PC and other electronic components connections. However, thisoff-line manual testing of calibration sheets assumes that the operatorcan properly manually identify and measure the test color sheets orpatches being tested in the correct order, from the correct machine.Once a color correction table is generated, it must be associated withthe correct printer, otherwise, a different printer will be recalibratedwith an incorrect correction table. An automatic, on-line, dedicatedspectrophotometer color correction system does not have these problemsor potential error sources.

[0046] It will be appreciated that although the specific embodimentherein is described with particular reference to such desirableapplications for calibrating and/or regularly re-calibrating colorprinters and/or refining color correction tables, that what is disclosedherein may also find various other applications in other color testingand correction systems and industries.

[0047] As discussed, in high quality color reprographic applications, itis highly advantageous to monitor and update system colorimetricperformance on-line and automatically through the use of an integratedspectrophotometer. That is, to have the printing device automaticallyfairly frequently generate calibration prints on otherwise normallyprinted sheets with color patches based on digital test patterngenerations, and to have a spectrophotometer in the printer output whichcan read those moving sheet printed color test patches accurately toprovide printed output color measurement signals, without manualintervention or printing. This requires a relatively low cost yet fast,accurate, and wide spectral range spectrophotometer capable ofeffectively operating in that environment, and under those conditions,without interfering with normal printing operations. That is, being ofsufficiently low cost such that this enhanced feature can be provided oncommercial color printers without substantially increasing the totalcustomer cost of those printers. That is not typical for conventionallaboratory spectrophotometers. The disclosed spectrophotometerembodiment may be positioned at any convenient location along the normalpaper path of a printing machine. It may even be fitted into the outputsheet stacker tray of various existing color printers.

[0048] A specific feature of the specific embodiment disclosed herein isto provide a color correction system for a color printer having anoutput path for moving printed color sheets, including printed testsheets with printed color test patches, in which a spectrophotometer ismounted adjacent to said printer output path for sensing the colorsprinted on said printed color test patches on said printed test sheetsas said printed test sheets are moving past said spectrophotometer insaid output path, and in which a limited plurality of illuminationsources are provided for sequentially illuminating said color testpatches with different illumination spectra, and a photodetector systemfor providing electrical output signals in response to the color of saidtest patches from said sequential illumination of said test patches byreflection of said illumination of said color test patches by saidillumination sources to illuminate said photodetector system; saidphotodetector system having a multiplicity of simultaneously illuminatedphoto-sites including at least three different sets of simultaneouslyilluminated photo-sites having at least three different spectralresponses providing at least three different said electrical outputsignals.

[0049] Further specific features disclosed herein, individually or incombination, include those wherein said photodetector system comprisesat least one low cost commercial photodetector chip designed for a partof a document color imaging bar and having at least three rows of smallclosely spaced photo-sites with integral red, green and blue colorfilters, respectively, to provide said at least three different spectralresponses with at least three different said electrical output signalsin parallel; and/or wherein said photodetector chip is modified to add aplurality of said simultaneously illuminated photo-sites which are broadspectral responsive photo-sites providing a fourth spectral responsedifferent from that of said photo-sites with integral red, green andblue color filters, and wherein at least one of said limited pluralityof illumination sources produces white light; and/or wherein saidlimited plurality of illumination sources comprises less thanapproximately five LEDs providing a corresponding limited number ofdifferent spectral illuminations, and a sequential actuation circuit forsaid LEDs; and/or a low cost broad spectrum spectrophotometer includinga limited plural number of illumination sources with different spectralilluminations arranged to illuminate a color test target area, asequential actuation circuit for sequentially actuation of said limitedplural number of illumination sources, and at least one low costcommercially available photodetector chip at least a portion of which isarranged to receive reflected light from said illuminated color testtarget area, said photodetector chip being a component part for adocument color imaging bar, and said photodetector chip having at leastthree rows of small and closely spaced multiple photo-sites withdifferent respective color filters of which at least a portion of eachof said three rows of multiple photo-sites are simultaneously exposed tosaid reflected light from said illuminated color test target to providesaid at least three different spectral responses with at least threedifferent electrical output signals in parallel; and/or wherein saidlimited plurality of illumination sources comprises less thanapproximately five LEDs providing a corresponding limited number ofdifferent spectral illuminations; and/or wherein said limited pluralityof illumination sources includes one broad spectrum white lightillumination source; and/or wherein said spectrophotometer is a part ofa color control system of a color printer with a printed sheets outputpath and is mounted adjacent to at least one side of the printed sheetsoutput path of said color printer and said illuminated color test targetarea is printed on a printed color test sheet printed by said printerand moving past said spectrophotometer in said printed sheets outputpath of said color printer; and/or wherein said limited plurality ofillumination sources comprises less than approximately five LEDsproviding a corresponding limited number of different spectralilluminations, which LEDs are mounted arrayed around said photodetectorchip and spaced from said color test target area to angularly illuminatesaid color test target area at substantially the same angle fromopposing directions; and/or wherein said limited plurality ofillumination sources are mounted in a substantially circular patternsurrounding said photodetector chip to define a central axis and arespaced from said color test target area to angularly illuminate saidcolor test target area at substantially the same angle from opposingdirections, and wherein said photodetector chip is aligned with saidcentral axis, and wherein a lens system is mounted on said central axisfor transmitting said illumination reflected from said color test targetarea to a limited area of said photodetector chip containing at least aportion of each of said three rows of said multiple photo-sites; and/orwherein said at least one low cost commercially available photodetectorchip is a component part for a document color imaging bar having atleast three rows of small closely spaced photo-sites with integral red,green and blue color filters, respectively, to provide said at leastthree different spectral responses with at least three differentelectrical output signals thereof in parallel; and/or a method of broadspectrum color measurement of a color test area comprising sequentiallyilluminating said color test area with a limited plural number ofdifferent spectra illuminations and sequentially measuring the reflectedillumination from said sequentially illuminated color test area byapplying said reflected illumination simultaneously to multiplephoto-sites of a multi-photo-site photodetector, which simultaneouslyexposed multiple photo-sites comprise at least three different sets ofphoto-sites with three different illumination responsive spectralresponses and three different parallel illumination responsive signaloutputs thereof; and/or wherein said limited plural number of differentspectra illuminations is provided by less than approximately five LEDsproviding a corresponding limited number of different spectralilluminations of said color test area; and/or wherein one of saidlimited plural number of different spectra illuminations is broadspectrum white light; and/or a low cost broad spectrum spectrophotometercomprising means for sequentially illuminating a color test area with alimited plural number of different spectra illuminations, and means forsequentially measuring the reflected illumination from said sequentiallyilluminated color test area by applying said reflected illuminationsimultaneously to multiple photo-sites of a multi-photo-sitephotodetector, which simultaneously exposed multiple photo-sitescomprise at least three different sets of photo-sites with threedifferent illumination responsive spectral responses and three differentparallel illumination responsive signal outputs thereof; and/or whereinsaid limited plural number of different spectra illuminations isprovided by three to four different LEDs providing a correspondinglimited number of different spectral illuminations, and a sequentialactuation circuit for said LEDs; and/or wherein said multi-photo-sitephotodetector is a low cost photodetector chip which is normally acomponent part for a document color imaging bar having at least threerows of small closely spaced photo-sites with integral red, green andblue color filters respectively to provide said at least three differentspectral responses with at least three different electrical outputsignals thereof in parallel; and/or including color test areadisplacement insensitive optics means; and/or a low costspectrophotometer comprising a broad spectrum white light illuminatorfor illuminating a color test target area and at least onemulti-photo-site photodetector, wherein said multi-photo-sitephotodetector is a low cost commercial photodetector chip which isnormally a component part of a document color imaging bar having atleast three rows of small closely spaced photo-sites with respectivered, green and blue color filters to provide at least three differentspectral responses of at least three different electrical outputsignals, said multi-photo-site photodetector being optically positionedto receive reflected light from said color test target area illuminatedby said broad spectrum white light illuminator; and/or including pluraldifferent spectra LED illuminators and a sequential LED actuatingcircuit; and/or including a lens system and wherein said photodetectorchip is oriented substantially in the plane of the image of saidreflected light through said lens system.

[0050] The disclosed system may be connected, operated and controlled byappropriate operation of conventional control systems. It is well knownand preferable to program and execute various control functions andlogic with software instructions for conventional or general purposemicroprocessors, as taught by numerous prior patents and commercialproducts. Such programming or software may of course vary depending onthe particular functions, software type, and microprocessor or othercomputer system utilized, but will be available to, or readilyprogrammable without undue experimentation from functional descriptions,such as those provided herein, and/or prior knowledge of functions whichare conventional, together with general knowledge in the software andcomputer arts. Alternatively, the disclosed control system or method maybe implemented partially or fully in hardware, using standard logiccircuits or single chip VLSI designs.

[0051] In the description herein, the term “sheet” refers to a usuallyflimsy (non-rigid) physical sheet of paper, plastic, or other suitablephysical substrate or print media for images, whether precut or web fed.A “copy sheet” may be abbreviated as a “copy,” or called a “hardcopy.”Printed sheets may be referred to as the “output.” A “print job” isnormally a set of related printed sheets, usually one or more collatedcopy sets copied from a one or more original document sheets orelectronic document page images, from a particular user, or otherwiserelated.

[0052] As to specific components of the subject apparatus, oralternatives therefor, it will be appreciated that, as is normally thecase, some such components are known per se in other apparatus orapplications which may be additionally or alternatively used herein,including those from art cited herein. All references cited in thisspecification, and their references, are incorporated by referenceherein where appropriate for appropriate teachings of additional oralternative details, features, and/or technical background. What is wellknown to those skilled in the art need not be described here.

[0053] Various of the above-mentioned and further features andadvantages will be apparent from the specific apparatus and itsoperation described in the example below, and the claims. Thus, thepresent invention will be better understood from this description of aspecific embodiment, including the drawing figures (approximately toscale, except for schematics) wherein:

[0054]FIG. 1 is a top view of one example or embodiment of aspectrophotometer incorporating one example of the present invention;

[0055]FIG. 2 is a cross-sectional view taken along the line 2-2 of thespectrophotometer of FIG. 1 shown measuring the color of a test patch ofa test sheet moving in an exemplary color printer output path;

[0056]FIG. 3 schematically shows one example of driver circuitry withwhich the LEDs of the exemplary spectrophotometer of FIGS. 1 and 2, or13, may be operated;

[0057]FIG. 4 shows one example of a banner or other test sheet which maybe printed by an exemplary color printer with plural color test patchesto be read by the spectrophotometer of FIGS. 1 and 2, with the differentcolors represented by their U.S. Patent Office standard black and whitecross-hatching symbols;

[0058]FIG. 5 is a schematic and greatly enlarged partial plan view of anexemplary silicon color image sensor array chip (part of a commerciallyavailable document imaging bar) utilized in the exemplaryspectrophotometer of FIGS. 1 and 2, with three rows of photosensor sitestransmissively filtered red, green and blue, respectively, in a knownmanner, for respectively sensing spectra in those three separate colors,and also showing an (optional) fourth row of photosensor sites withoutfilters for white light sensing, with the area defined by the ellipseillustrated thereon representing an exemplary area of this sensor arraychip being illuminated by LED source light reflected by a test target;

[0059]FIG. 6 schematically shows in a plan view one example of anotherwise conventional color printer, shown printing the color testsheets of FIG. 4 and sequentially reading those test sheets with thespectrophotometer of FIGS. 1 and 2 as the test sheet are moving normallyin the normal output path of this printer, with the spectrophotometershown here mounted at one side of that sheet output path opposite froman opposing calibration test target surface;

[0060]FIG. 7 shows in a plot of wavelength (horizontal) versus relativeresponse (vertical) the four exemplary spectral responses of theexemplary image sensor array chip of FIG. 5, respectively for itsunfiltered sensors (the solid line), blue filtered sensors (the dashedline), green filtered sensors (the dot-dashed line) and red filteredsensors (the dotted line);

[0061]FIG. 8 is similar to FIG. 7 but shows superimposed on the curvesof FIG. 7 the spectral outputs of four different exemplary LEDillumination sources which may be integral to the exemplaryspectrophotometer of the above Figs (as described and shown in the tablebelow), namely a white LED (the dash-long-dash line), a 430 nm LED (thethin line), and 505 nm LED (the line of squares), and a 595 nm LED (thedash-dot-dot dash line);

[0062]FIGS. 9, 10, 11 and 12, respectively, sequentially show thecombined response of all four different sensors of the chip of FIG. 5 assequentially exposed to illumination from only one of the four differentLEDs of FIG. 8, namely, in FIG. 9 the white LED, in FIG. 10 the 430 nmLED, in FIG. 11 the 505 nm LED, and in FIG. 12 the 595 nm LED; and

[0063]FIG. 13 illustrates an alternative embodiment of thespectrophotometer architecture of FIG. 2 in which the position of theLEDs are reversed with the positions of FIG. 5 photodetector chips toprovide improved insensitivity to angular displacements of the testtarget surface, as in the cross-referenced commonly filed application.

[0064] We will now refer in further detail to the specific exemplaryembodiment of a color sensing system 10 with a spectrophotometer 12 or12′(FIG. 13) as shown in the above-described Figures, noting first FIGS.1-4. As variously previously discussed, this spectrophotometer 12embodiment (or alternatives thereof) is particularly suited to be partof a highly effective yet economical on-line or “real time” colorprinting color calibration or correction system, which can regularlymeasure the actual colors currently being printed by a color printersuch as 20 of FIG. 6 on banner or other printed test sheets such as 30of FIG. 4, as compared to the intended or selected, or “true” colors ofthe electronic document images being inputted to the printer 20 forprinting. However, as also noted above, the disclosed spectrophotometer12 or 12′ is not limited to that disclosed combination, application orutility.

[0065] In these spectrophotometer 12 or 12′ embodiments, only a few LEDs(e.g., only three or four, such as D1, D2, D3 and D4) of appropriatedifferent color spectral emission outputs need be utilized tosequentially illuminate an area 35 of the exemplary color test targets31 on the exemplary test sheets as in FIG. 4. Furthermore, in thespectrophotometer 12′ the reflected illumination level is not detectedby a single photocell. Instead, it is detected by one or more low costcolor image sensor arrays with multiple spectral response photo-sites,such as chip 14, as in the example of FIG. 5, having rows of closelyadjacent plural color sensors (photo-sites D12F, D12E, D12C and D12D)with respective plural different integral color filtering (none, blue,green and red) providing plural different spectral sensitivities, andplural parallel output signals, rather than a single output signal froman individual (single cell) photosensor. The respective different coloroutput LEDs D1, D2, D3 and D4 may be switched on in a predeterminedsequence (as shown in FIG. 3 or otherwise) to provide plural specificdifferent spectral reflectance measurements within the visiblewavelengths, as illustrated in FIGS. 7-12. This provides a fast and lowcost general color sensing solution.

[0066] If desired, those spectral measurements of an area of a testtarget may be converted to provide a true broad reflectance spectra,through known or other reconstruction and extrapolation algorithms. Boththe number and spectra of the LED illuminators and the photosensor sitesmay be varied, where appropriate, and are not necessarily limited to thespecific numbers and specific wavelengths of this specific embodimentexample.

[0067] It will be noted especially with respect to these descriptions ofimaging chips, that the terms “photosensor sites,” “photo-sites,”“photosensitive cells,” “cells,” “detectors,” or “sensors” are variouslyused interchangeably in descriptions herein, as in the art, unlessotherwise indicated.

[0068] As previously noted, commercial mass-produced low cost documentimaging bars are typically formed by edge butting together a pluralityof individual imaging chips, each having multiple tiny and closelyspaced photo-sites, as schematically shown in the FIG. 5 enlarged viewexample of such chip 14. Typically, each such chip 14 has three rows ofsuch photo-sites (D12D, D12C and D12E here) manufactured with integralfilters for red, green and blue, respectively. Also, each chip 14typically has integrated electronics; sample and hold circuitry, etc.The spectrophotometer 12 desirably utilizes at least one (or more, as in12′, depending on the spectrophotometer design) of these low costindividual imaging chips 14. It is suggested here that these chips 14may be obtained from a manufacturer before they are fastened togetherinto a document imaging bar.

[0069] As one example of such a known document imaging bar, it may bemade from twenty of such individual imaging chips 14, with each chip 14being 16 mm long. Each such chip can read 400×660 pixels, provided by248 photosensitive cells, with a 63.5 micro-meter pitch between cells.The cells are in three parallel rows, with filters for red, green andblue in the respective rows, as shown in the example of FIG. 5. Thesechips are made with integral electrical leads and connecting electronicsalready provided to all of these 248 photo-sites.

[0070] If desired, and as also illustrated in the FIG. 5 example,another such row of photo-sites, D12F, may be added to these chips, forwhite light (broad spectrum) sensing, by a relatively simplemodification. That is, by simply adding one more such parallel row ofcells in the same silicon semiconductor manufacturing steps (orotherwise) to provide another row of otherwise identical or similarphoto-sites, but having no color filtering layer formed over the cells.Alternatively, a different filter may be superimposed on the photo-sitesof that added fourth row. Alternatively, the chip may be made with thesame existing three rows of cells, but with every fourth cell in eachrow made without any filter. Or, every fourth cell in each row may bemade a different filter. Some aperturing (exposed area reduction, suchas by partial masking) may also be provided if desired for theunfiltered cells.

[0071] The cost of a suitable image sensor chip, as is, or modified asdescribed, may be considerably lower than a non-commercial photosensor.It can also provide a much higher level of circuit integration. Thus, amuch more cost-effective spectrophotometer can be made therefrom thanfrom individual photosensors, and a number of parallel sensing outputscan be provided.

[0072] As indicated above, the exemplary color image sensor chip 14 maydiffer somewhat from a conventional document color image sensor array orbar in that some of the photo-sites (D12F) on the color image sensorarray may be left uncovered, without any color filter layers. By doingso, a fourth, broadband, spectral measurement is enabled from thoseunfiltered photo-sites along with the three different spectralmeasurements that the chip normally provides from its three differentlycolored filter covered photo-sites D12E, D12C and D12D. As noted, whilecommercially available color image sensor array chips typically havethree rows of photo-sites that are coated with three different colorfilter layers; red, green and blue, thus providing a three color spectrameasurement capability, these same sensor array chips can be modified atlow cost by simple modifications to provide an additional fourthspectral measurement capability. That is, modified so that some of thephoto-sites are not color filtered. A broad spectrum illuminationsource, such as a white light LED, may be used therewith in aspectrophotometer configuration, as further described herein.

[0073] As shown herein, a spectrophotometer with a suitable combinationof a relatively small number of plural LEDs plus plural simultaneouslyexposed photo-sites, with an appropriate LED switching sequence to turnthe LEDs on and off, can rapidly provide a large number of test targetcolor measurements. As the number of measurements is so increased, thecolor measurement capability also becomes more accurate.

[0074] Depending on the particular color correction or calibrationsystem needs, different numbers of LEDs can be used. However, it hasbeen found that only a few LEDs having spectral output covering thesensitivity ranges of only two or more different types of photo-sites,plus a white LED or other light source, can provide a low totalcomponents count, and thus a lower cost, spectrophotometer, yet providea relatively large number of spectra measurements.

[0075] This can be further understood by reference to the exemplaryspectral curves shown in FIGS. 7-12 and their above Fig. descriptions.In FIGS. 7-12 the respective curves corresponding to exemplary LEDs havebeen labeled with the same reference numbers of the exemplary LEDs, D1,D2, D4 or D5, as those same reference numbers are used in the exemplarycircuit of FIG. 3, for convenience and illustrative clarity. Incontrast, D12 in FIG. 3 is schematically representing the combined inputof plural photo-sites of the color sensing chip 14.

[0076] As noted, FIG. 5 is a schematic and greatly enlarged portion of aexemplary color image sensor array chip 14 which may be utilized in theexemplary spectrophotometer 12 of FIGS. 1 and 2. Show in FIG. 5 is anexemplary illuminated area 34 thereof. This area 34 is illuminated byLED illumination reflected from a illuminated test target 31 area 35 inFIGS. 2 and 4, and through the lens system 13 of FIG. 2, tosimultaneously illuminate multiple photo-sites in the four rows of thechip 14. Those simultaneously illuminated photo-sites include the red,green, blue photo-sites D12D, D12C and D12E, and also the unfilteredphoto-sites D12F if they are provided on the chip 14.

[0077] For the alternative embodiment 1240 of FIG. 13, a FIG. 4 circulartarget area 35 (dashed line) is illuminated via lens 13′. It's reflectedlights reflect through lenses 18′ and 19′ to a circular area 34′ (dashedline) in FIG. 5 on all of the chips 14.

[0078] The Table below further shows the number of spectral measurementsthat can be made with examples of combinations of different numbers ofspecific LEDs and an image sensor chip 14 with different photo-sitefilters: Number of Spectral Measurements With 3 Color With 4 Color (R,G, B (R, G, B filters LEDs filters) + no filter) Types Number ImageSensor Image Sensor White 1 3 4 White, + 595 nm 2 5-6 7-8 or 505 nmWhite, 595 nm, 3 7-9 10-12 505 nm White, 595 nm, 4  8-12 12-16 505 nm,430 nm

[0079] It may be seen from the last (bottom line) example of this Tablethat with a four color image sensor chip 14 (with unfiltered photo-sitesin addition to red, green and blue filter photo-sites), that at least 4,3, 3 and 2 (12 total) sets of spectral measurements can be obtained bydetecting a color test target 31 illumination by only four LEDs (white,595 nm peak, 505 nm peak and 430 nm peak). Thus, one can see that atleast 12 spectral combinations can be measured using a spectrophotometerhaving only four LEDs and a single, low cost, multipixel (multiplephoto-sites) image sensor array (chip) 14. Additionally using thelower-level signals (e.g., D3 in FIGS. 10 and 12) up to 16 spectralcombinations can be measured in this example.

[0080] Integration times used with various rows of the image sensorarray chip 14 can be independently controlled to match the LED powerlevels to get suitable output signals from the sensor array.

[0081] As discussed, some of the photo-sites in one or more of theserows are desirably left uncovered (with no color filters) to get fourspectral outputs from an otherwise conventional three row image sensorarray. In general, the photo-sites that are not covered with colorfilters will provide a much larger output signal than those that arecovered with filters. To compensate, part of the sensing area of theseuncovered (unfiltered) photo-sites can be optionally coated inmanufacturing with an opaque material or multiple layers of all threecolor filter layers to reduce their light sensitivity.

[0082] Any or all of the outputs of the sensor chip 14 may, of course,be calibrated/reconstructed to provide true reflectance values. Forexample, as in the above-cited U.S. application Ser. No. 09/562,072,filed May 1, 2000 by Lingappa K. Mestha, et al., entitled “System andMethod for Reconstruction of Spectral Curves, Using Measurements from aColor Sensor and Statistical Techniques,” Attorney Docket No. D/99803.

[0083] It may be seen that a novel spectrophotometer 12 is disclosedwhich combines the spectral differentiation capabilities of a low costplural spectra image sensor 14 with the spectral outputs of a relativelysmall number of different LEDs to enable a cost effective, highperformance, spectrophotometer. The following and/or other advantagesmay be provided: multiple measurements can be made and outputted inparallel corresponding to three or four different color image sensoroutputs in parallel; cost can be reduced by reducing the number of LEDsand having lower detector and detector electronics costs; and theintegration time of the three or four rows of a three or four row imagesensor array can be adjusted independently to match the power levels ofdifferent LEDs.

[0084] Referencing the first line of the above table, an alternativeapplication, function, or option is to turn on, and leave on, only thewhite illumination source, for all of the color test patches being readat that time, to provide a “calorimeter” function of RGB values from thechip 14 outputs.

[0085] Describing now the exemplary operation of the exemplary colorsensing system 10 using an exemplary spectrophotometer 12, certainaspects thereof are also described in above-cited references and theabove cross-referenced U.S. application Ser. No. 09/535,007, filed Mar.23, 2000, by Fred F. Hubble, III and Joel A. Kubby, Attorney Docket No.D/99511i.

[0086] In the illustrated example here, the spectrophotometer 12 may beutilized with circuitry, such as that of FIG. 3, or otherwise, toaccurately read reflected light from one or more different color testpatches such as 31 printed on moving color test sheets 30 such as thatshown in FIG. 4. The test sheets 30 may be conventionally printed onvarious print media such as conventional print papers or plastics,preferably the same print media as the planned or concurrent print jobitself. The color test patches 31 may be printed in the same manner andby the same print apparatus as the regular print jobs by any of variousdifferent conventional color printer or printing systems, of which thexerographic printer 20 of FIG. 6 is merely one example.

[0087] As will be further described, the disclosed spectrophotometer 12can accurately read the colors of the test patches 31 even though thetest sheets 30 are variably spaced from the spectrophotometer 12 duringtheir color measurements, and are moving. Thus, the color measurementsare not affected by normal variations in sheet surface positions in anormal paper path of a printer. This allows the simple mounting of thespectrophotometer 12 at one side of the normal printed sheets outputpath 40 of the printer 20 (or various other color reproduction systems).

[0088] Briefly first describing the exemplary color printer 20 of FIG. 6in more detail, it is schematically illustrating an otherwiseconventional xerographic laser color printer, details of various ofwhich will be well known to those skilled in that art and need not bere-described in detail herein. Examples of further descriptions are inthe above-cited Xerox Corp. U.S. Pat. No. 5,748,221, etc., and other artcited therein. A photoreceptor belt 26 is driven by a motor M and laserlatent imaged or exposed by a ROS polygon scanning system 24 aftercharging (or an LED bar). The respective images are developed by a blacktoner image developer station 41 and/or one or more of three differentcolor toner image developer stations 42A, 42B, 42C. The toner images aretransferred at a transfer station 32 to sheets of copy paper fed from aninput tray stack 36. Where one or more test sheets 30 are being printedinstead of normal document images (at times, and with color tests,selected by the controller 100), each such test sheet 30 may be fed fromthe same or another sheet supply stack 36 and its test imagestransferred in the normal manner. The test sheet 30 is then outputtedthrough the fuser to the same normal output path 40, as if it were anyother normal sheet being normally color printed. The test sheets 30 maybe dual mode sheets also serving as banner sheets for print jobseparations, with typical printed banner sheet information, such as oneor more of the user's name, the document title, the date and time, orthe like.

[0089] The spectrophotometer 12 here is mounted at one side of thatoutput path 40 (or, it could even be mounted over the output tray 44) tosense the actual, fused, final colors being printed. Thespectrophotometer output signals provide the input for the on-line colorsensing and correction system 10, here with a microprocessor controller100 and/or interactive circuitry and/or software. The controller 100,and sheet sensors along the machine 20 paper path, conventionallycontrols the feeding and tracking of sheet positions within the printerpaper path. The controller 100 and/or a conventional sensor forfiduciary marks 33 or the like on the test sheet 30 can provide controlor actuation signals to the spectrophotometer 12 circuitry for thespectrophotometer 12 to sequentially test or read the colors of each ofthe test patches 31 on the test sheet 30 as that test sheet 30 movespast the spectrophotometer 12 in the output path 40. The test patches 31can be variously located and configured, as blocks, strips, orotherwise, of various digitally selected solid color images.

[0090] Thus, in the disclosed embodiment, plural test sheets 30 of paperor other image substrate material being printed by the color printer 20can be automatically printed with pre-programmed plural test patches 31of one or more defined colors, preferably with associated simplefiduciary marks for signaling the reading location of each colored testpatch on the test sheet. Each test sheet 30 moves normally past thefixed position spectrophotometer 12, which is unobstructedly mounted atone side of the normal post-fuser machine output path 40 to bothilluminate and view sheets passing thereby. This is in contrast to thoseprior systems requiring removing and holding a test sheet still, andmoving a standard contact calorimeter or spectrophotometer over the testsheet.

[0091] It will be seen in FIGS. 1 and 2 that the exemplary compactspectrophotometer 12 shown in that example has only four different colorsampling illumination sources, provided by four commonly target-aimedbut sequentially operated LEDs, D1 through D4, each with different colorspectrum range outputs. Each LED output may have the same simplecondenser lens, such as 18 and 19 in FIG. 2, for directing the lightfrom the respective LED onto the same test target area, as shown by theelliptical illuminated area of FIG. 4. Color filters for the LEDs, suchas 16 and 17, may be provided in some cases if desired to furthercontrol the spectral range, but are not essential. The normal targetarea in the system 10 embodiment herein is an area of a printed colortest patch or patches 31 on the sheet of paper being otherwise normallyprinted and outputted. An alternate or calibration target area could bean unprinted area of the test paper sheet, or a white, grey, black orother color standardized test tile or surface automatically solenoid (ormanually) inserted into the effective field of view of thespectrophotometer.

[0092] As particularly shown in FIG. 2, the test target illumination byany one of the LEDs provides a variable level of light reflected fromthat target depending on the colors of the test patch and the selectedillumination source. A portion of that reflected light may collected bythe single central lens 13 and focused by that lens 13 onto singlephotosensor chip 14 to expose an array of multiple photo-sites, with 3or 4 different colors of filtering, as described herein. FIG. 2illustrates, with dashed line light rays, both the LED illumination andthe focusing by the projection lens 13 (a simple two-element optic inthis example) of three exemplary target points A, B and C onto the focalplane of lens 13 as C′, B′ and A′.

[0093] Although conventional glass or plastic lenses are illustrated inthe spectrophotometer 12 of FIGS. 1 and 2, it will be appreciated thatfiber optics or selfoc lenses could be utilized instead in otherapplications. Fiber optics may be used to conduct the illumination fromthe LEDs. Also, a collecting fiber optic may be used if it is desired,for example, to space the detecting photosensor array remotely from thefocal plane of the lens 13.

[0094] As utilized in this disclosed embodiment of an on-line colorsensing system 10, this low cost spectrophotometer 12, as mounted in theprinter 20 copy sheet output path 40, can thus be part of a colorcorrection system to automatically control and drive to color printingaccuracy the printer 20 CMYK color generation with a small number ofprinted test sheets 30. The color correction system can sequentiallylook at a relatively small series of color test patterns printed on copysheets as they are outputted. One or more mathematical techniques forcolor error correction with multiple spectrophotometer-detected outputcolor signals for each color patch as input signals can provide for agreatly reduced number of required printed test patches, as shown in theabove-cited references. That is, by recording the detector arraymultiple outputs when a test patch is successively illuminated by eachindividual LED, the reflectance of the test patch as a function ofdifferent wavelengths can be determined, and that reflectance of thetest patch, as a function of different wavelengths, can be extrapolatedor interpolated over the entire visible spectra.

[0095] An accurate color control system, as disclosed herein, can thusregularly or almost constantly provide for testing and storing currentmachine color printing responses to color printing input signals (anup-to-date model) for remapping LAB (or XYZ) “device independent” colorinputs (for later conversion to device dependent RGB or CMYK color spacefor printing). That information can also be profiled into a system ornetwork server for each different machine (and/or displayed on a CRTcontroller for color manipulation).

[0096] As further described in the above cross-referenced applications,the exemplary spectrophotometer 12 shown in FIGS. 1 and 2 may be, and ishere, desirably optically designed to be insensitive to the separationbetween the sensing head and the test patch target sheets, by selectingthe magnification of the target optic 13 to be approximately 1:1. (Anexemplary focal length of the lens system 13 may be about 32 mm.) Thedegree of spatial insensitivity this provides allows non-contactmeasurements of moving printed sheets having variable distance spacingsfrom the spectrophotometer 12, and thus allows for an unobstructedprinter paper path. This is further explained in more detail in theabove cross-referenced application Attorney Docket No. D/99511i.However, there may be some applications of this spectrophotometer inwhich displacement invariance is not critical, on which case lenses maynot be required.

[0097] To provide a desired “overfill,” to avoid any effect of anenlarged exposure area on the imaging chip 14 from an increased targetspacing, the connecting circuitry may be set to ignore or threshold anyonly partially exposed cells (photo-sites) and/or may be set to onlylook at a fixed minimum number of centrally exposed cells, ignoring anysignals from outer cells even if those outer cells are being illuminatedby light reflected from the target.

[0098] With the differently color filtered cells of the FIG. 5 chip 14,the connecting circuitry can also tell which cells are being exposed towhich color from an illuminated test patch. Thus, as shown in FIG. 4,plural color test patches can be simultaneously illuminated, yet can bedesirably utilized for increased data. That is, more than one individualcolor test patch can be tested at a time by this spectrophotometer 12.However, that is not required here. Exposing (sensing) only one singlecolor test patch at a time, as shown in the above cross-referencedapplications, and several cited references, may be utilized. Themultiple signals provided from multiple photo-sites with pluraldifferent color filters may be utilized for analyzing the reflectedlight from either type of test target.

[0099] In the spectrophotometer embodiment 12 of FIGS. 1 and 2 the testpatch 31 illuminations are at 45 degrees to the surface of the media onwhich the color test patch is printed, and the sensing system isdetecting flux diffusely scattered from the (so-illuminated) test patchat 90 degrees (perpendicular to) that same color test patch surface.However, as will be discussed later below, and shown in FIG. 13, it isnot limited to that configuration.

[0100] Various different technologies, architectures, and/or componentsmay be used. For example, as in FIG. 13, all of the LEDs D1, D2, D3, D4may be provided by a single on-board chip or board. In thatarchitecture, an appropriate selection of LED die with differentwavelengths covering the visible spectrum may be formed in a circularpattern on a PWB.

[0101] The flux from each LED may be collimated and centrally directedto be applied to the same test patch under the center of thespectrophotometer in both 12 and 12′. That position is also on thecenter axis of the lens 13 or 13′, which lens 13 or 13′ is located inthe center of the ring or circle of LEDs, as shown in FIG. 1. Thisenables in FIG. 2 an image of the illuminated patch to be projected ontoa single integral detector array 14 on that same central axis. The lens13′ in FIG. 13 may have an IR filter 13A.

[0102]FIG. 3 is a schematic or block diagram of an exemplary LED driverfor the spectrophotometer 12 of FIGS. 1 and 2, or 12′ of FIG. 13,portions of which are generally identified here for convenience as partof the controller 100, even though it can be, in whole or in part, aseparate circuit, desirably having a single driver chip or die for allof the LEDs in the spectrophotometer itself. In response to regulartiming signals from the circuit 110 labeled “LED Drive” here, each LEDis pulsed in turn by briefly turning on its respective transistor driverQ1 through Q4, by which the respective different spectra LEDs D1 throughD4 are turned on by current from the indicated common voltage supplythrough respective resistors R1 through R4. Four different exemplarylight output colors of the four respective LEDs are indicated in FIG. 3by the legends next to each of those LEDs. Thus, each LED may besequenced one at a time to sequentially transmit light though thecondenser lenses such as 18 and 19 shown in FIG. 2, and 13′ in FIG. 13.

[0103] While the LEDs in this example are turned on one at time insequence, it will be appreciated that the system is not limited thereto.There may be measurement modes in which it is desirable to turn on morethan one LED or other illumination source at once on the same targetarea.

[0104] The relative reflectance of each actuated LEDs color orwavelength may measured by using conventional circuitry or software foramplifying and integrating the respective outputs of the photodiodedetector chip 14 array of photo-sites, which also has integral sampleand hold circuitry. As discussed, the LED pulsing and detector samplingrate is sufficiently non-critical and rapid for sampling each ofmultiple reasonable size color test patches on a normal size copy sheetmoving by the spectrophotometer even for a high speed printer movingsheets rapidly through its paper path. However, by briefly pulsing thecommon LED driver voltage source to provide brief LED drive currents ata level above what is sustainable in a continuous current mode, evenhigher flux detection signals can obtained and the test patch can thusbe interrogated in a shorter time period. In any case, by thresholdingand/or integrating the output signals, enhanced signal-to-noise ratioscan be achieved. It may be seen that FIG. 3 shows merely one example ofa relatively simple and straightforward circuit. It, or variousalternatives, can be readily implemented in an on-board hybrid chip orother architecture.

[0105] An additional conventional LED light emitter and detector may beintegrated or separately mounted to detect black fiduciary or timingmarks 33 printed on the test sheet 30 of FIG. 4, thereby providing anenable signal for illumination and reading within the respective colortest patch areas. Those fiduciary marks 33 indicate the presence of anadjacent test patch 31 in the field of view of the spectrophotometer 12.However, it will be appreciated that with sufficiently accurate sheettiming and positional information already conventionally provided in theprinter 20 controller 100, or provided by spectrophotometer output data,such fiducial marks 33 may not be needed. These fiducial marks 33 may bealong side of their corresponding color test patch or patch area asshown in FIG. 4, or in between each (spaced apart) color test area.I.e., the fiducial marks may be parallel to, or in line with, the testpatches in the direction of motion of the test sheet relative to thespectrophotometer.

[0106] Individual calibration for each of the spectrophotometer's LEDspectral energy outputs may be done by using a standard white (or other)tile test target of known reflectivity for the spectrophotometer toconvert each LED measurement to absolute reflectance values. Thiscalibration can be done frequently, automatically, and without removingthe spectrophotometer from the printer with a standard white calibrationtile test surface, such as 47 shown in FIG. 6, being manually, orpreferably automatically (as by a solinoid), placed oppositely from thespectrophotometer 12, on the other side of the paper path 40 but in thefield of view of the photosensor array and its lens system 13. Thus,during any selected, or all, of the inter-sheet gaps (the normal spacingbetween printed sheets in the sheet path of the printer) a recalibrationcan be carried out without having to move or refocus thespectrophotometer.

[0107] This or other calibration systems can convert the individualoutput energies of the respective LEDs at that point in time on thecalibration tile 47 into respective individual reflectance measurementvalues from the photosensor array D12. That calibration data can then beelectronically compared to previously stored standard characteristicsdata in the controller 100, or elsewhere, to provide calibration datafor the spectrophotometer 12, which may be used for calibration of itsother, color test patch generated, data. The calibration data can alsobe used to adjust the individual LED output energies to compensate forLED aging or other output changes, by adjusting the applied current orvoltage (if that is individually programmable) or by increasing therespective turn-on times of the LEDs, where the photodetector D12 outputsignal is being integrated, as in this embodiment.

[0108] Initial spectrophotometer calibration data may be stored in anintegral PROM IC shipped with the spectrophotometer, if desired.Alternatively, LED output initial calibration data may be programmedinto the software being used to analyze the output of thespectrophotometer in other known manners, such as loading it into thedisc storage or other programmable memory of the printer controller 100or system print server.

[0109] It is well known to use conventional optical filters of differentcolors for each of respectively different color LED spectrophotometertarget illumination sources. In particular, it is well known to use suchcolor filters to exclude secondary emissions from LEDs, and/or tofurther narrow the output spectra of LED illumination sources. Suchcolor filters are believed to be used for that purpose in some “AccuracyMicrosensors”™ LED based commercial products, for example. However, itwill be further appreciated by those skilled in this art that such colorfilters are not needed for those LEDs having sufficiently narrowbandwidths or for those LEDs which do not have secondary emissions thatneed to be suppressed. Therefor, filters may, but need not, be employedfor the LEDs of the subject spectrophotometer.

[0110] It will also be noted that spectrophotometers have been madeusing illumination sources other than LEDs. For example, multipleelectroluminescent (EL) emitters with filter and active layers as in HPU.S. Pat. No. 5,671,059, issued Sep. 23, 1997, or incandescent lamps.Also, as noted in the introduction, white (instead of narrow spectrum)LED illuminators and plural sensors with different color filters aredisclosed in EP 0 921 381 A2 published Sep. 6, 1999 for a color sensorfor inspecting color print on newspaper or other printed products.

[0111] In the particular spectrophotometer embodiment 12 configurationshown in FIG. 2, as described, the photosensor (detector) is on thecentral or zero axis of the spectrophotometer to receive reflected lightperpendicularly (at 90 degrees) from the illuminated area of the testtarget, and that illumination is by plural LEDs spaced around thatcentral axis aimed at 45 degrees to the test target. As an alternativeembodiment, as shown in FIG. 13, a desirable alternative is to reversethose component positions in the spectrophotometer 12′ shown there. Thatis, to put all of the plural different color emission LEDs together inone central unit, board, or chip, projecting light in parallel along thecentral or zero axis of the spectrophotometer 12′ at 90 degrees to thetest target (e.g., the color patch on the moving sheet of paper), so asto provide a substantially circular, 35′ rather than elliptical, 35,illuminated area of the test target 31. Also suggested, and shown inFIG. 13, is to put one or more photo-sensor chips 14 physically orientedat 90 degrees to the test target plane to receive the reflected lightfrom the test target optically oriented at 45 degrees to the testtarget. This change from a 45-0 degree system to a 0-45 degree systemhas been discovered to reduce measurement errors from test targetangular or azimuthal misalignment relative to the spectrophotometer 12′.

[0112] By way of further explanation of the above FIG. 13 alternative,in a typical printer paper path with spaced baffles the angle of thetest paper sheet surface relative to the central axis of thespectrophotometer can vary somewhat, for various reasons. By having allthe LEDs centrally located, their illumination pattern on the testtarget may be formed from rays that hit the target at approximately 90degrees, i.e., normal to the target. This will produce a circular ornearly circular irradiance pattern on a selected area of the target whenthe target surface is at 90 degrees thereto, as intended. When thetarget surface deviates from 90 degrees, by factors such as paper leador trail edge curl, paper buckle, sensor mounting misalignment, or othereffects, this LED irradiance pattern becomes only slightly elliptical,with an area larger than the circle by the factor 1/cos(theta), wheretheta is the deviation from 90 degrees. For example if the incidentangle were to become 93 degrees, then theta would be 3 degrees, the areaof the irradiance would be A/cos(3)=1.001A, where A was the selectedilluminated area. The flux reflected from the target and collected bythe detectors is proportional to the irradiance. Since it may be seenthat the irradiance (energy per unit area) varies very little for this 3degrees example, only by 0.001, the signals from the detectors likewisevary very little.

[0113] An additional, if less significant, feature in improvingspectrophotometer accuracy for variable target angles with this abovealternative embodiment is to provide, in addition to the above, theaveraging of the outputs of the plural photodetectors which are viewingthe irradiance area from different positions around it, such as with anarrangement of photosensors similar to the FIG. 1 arrangement of LEDs,so as to average the varying angular and/or azimuthal reflectivity ofthe target area, and thus further increase the insensitivity to angularmisalignment with the target area. In the above example of a 3 degreetilted target surface, the detector on one side of the spectrophotometercentral axis will view the illuminated target area at 45 minus 3degrees, while the detector on the opposite side of thespectrophotometer will view the same illuminated area at 45 plus 3degrees, but their output signals may be averaged. It will beappreciated that these plural spaced detectors may desirably be low costsingle chip, multi-pixel, plural color, photo-detectors 14, such asthose described in detail in this application.

[0114] While the embodiment disclosed herein is preferred, it will beappreciated from this teaching that various alternatives, modifications,variations or improvements therein may be made by those skilled in theart, which are intended to be encompassed by the following claims.

What is claimed is:
 1. A color correction system for a color printerhaving an output path for moving printed color sheets, including printedtest sheets with printed color test patches, in which aspectrophotometer is mounted adjacent to said printer output path forsensing the colors printed on said printed color test patches on saidprinted test sheets as said printed test sheets are moving past saidspectrophotometer in said output path, and in which a limited pluralityof illumination sources are provided for sequentially illuminating saidcolor test patches with different illumination spectra, and aphotodetector system for providing electrical output signals in responseto the color of said test patches from said sequential illumination ofsaid test patches by reflection of said illumination of said color testpatches by said illumination sources, to illuminate said photodetectorsystem; said photodetector system having a multiplicity ofsimultaneously illuminated photo-sites including at least threedifferent sets of simultaneously illuminated photo-sites having at leastthree different spectral responses providing at least three differentsaid electrical output signals.
 2. The color correction system of claim1, wherein said photodetector system comprises at least one low costcommercial photodetector chip designed for a part of a document colorimaging bar and having at least three rows of small closely spacedphoto-sites with integral red, green and blue color filtersrespectively, to provide said at least three different spectralresponses with at least three different said electrical output signalsin parallel.
 3. The color correction system of claim 2, wherein saidphotodetector chip is modified to add a plurality of said simultaneouslyilluminated photo-sites which are broad spectral responsive photo-sitesproviding a fourth spectral response different from that of saidphoto-sites with integral red, green and blue color filters, and whereinat least one of said limited plurality of illumination sources produceswhite light.
 4. The color correction system of claim 2, wherein saidlimited plurality of illumination sources comprises less thanapproximately five LEDs providing a corresponding limited number ofdifferent spectral illuminations, and a sequential actuation circuit forsaid LEDs.
 5. A low cost broad spectrum spectrophotometer including alimited plural number of illumination sources with different spectralilluminations arranged to illuminate a color test target area, asequential actuation circuit for sequentially actuation of said limitedplural number of illumination sources, and at least one low costcommercially available photodetector chip at least a portion of which isarranged to receive reflected light from said illuminated color testtarget area, said photodetector chip being a component part for adocument color imaging bar, and said photodetector chip having at leastthree rows of. small and closely spaced multiple photo-sites withdifferent respective color filters, of which at least a portion of eachof said three rows of multiple photo-sites are simultaneously exposed tosaid reflected light from said illuminated color test target to providesaid at least three different spectral responses with at least threedifferent electrical output signals.
 6. The low cost broad spectrumspectrophotometer of claim 5, wherein said limited plurality ofillumination sources comprises less than approximately five LEDsproviding a corresponding limited number of different spectralilluminations.
 7. The low cost broad spectrum spectrophotometer of claim5, wherein said limited plurality of illumination sources includes onebroad spectrum white light illumination source.
 8. The low cost broadspectrum spectrophotometer of claim 5, wherein said spectrophotometer isa part of a color control system of a color printer with a printedsheets output path and is mounted adjacent to at least one side of theprinted sheets output path of said color printer and said illuminatedcolor test target area is printed on a printed color test sheet printedby said printer and moving past said spectrophotometer in said printedsheets output path of said color printer.
 9. The low cost broad spectrumspectrophotometer of claim 5, wherein said limited plurality ofillumination sources comprises less than approximately five LEDsproviding a corresponding limited number of different spectralilluminations, which LEDs are mounted arrayed around said photodetectorchip and spaced from said color test target area to angularly illuminatesaid color test target area at substantially the same angle fromopposing directions.
 10. The low cost broad spectrum spectrophotometerof claim 5, wherein said limited plurality of illumination sources aremounted in a substantially circular pattern surrounding saidphotodetector chip to define a central axis and are spaced from saidcolor test target area to angularly illuminate said color test targetarea at substantially the same angle from opposing directions, andwherein said photodetector chip is aligned with said central axis, andwherein a lens system is mounted on said central axis for transmittingsaid illumination reflected from said color test target area to alimited area of said photodetector chip containing at least a portion ofeach of said three rows of said multiple photo-sites.
 11. The low costbroad spectrum spectrophotometer of claim 5, wherein said at least onelow cost commercially available photodetector chip is normally acomponent part for a document color imaging bar, having at least threerows of small closely spaced photo-sites with integral red, green andblue color filters respectively to provide said at least three differentspectral responses with at least three different electrical outputsignals thereof in parallel.
 12. A method of broad spectrum colormeasurement of a color test area comprising sequentially illuminatingsaid color test area with a limited plural number of different spectrailluminations and sequentially measuring the reflected illumination fromsaid sequentially illuminated color test area by applying said reflectedillumination simultaneously to multiple photo-sites of amulti-photo-site photodetector, which simultaneously exposed multiplephoto-sites comprise at least three different sets of photo-sites withthree different illumination responsive spectral responses and at leastthree different illumination responsive signal outputs thereof.
 13. Themethod of broad spectrum color measurement of a color test area of claim12, wherein said limited plural number of different spectrailluminations is provided by less than approximately five LEDs providinga corresponding limited number of different spectral illuminations ofsaid color test area.
 14. The method of broad spectrum color measurementof a color test area of claim 12, wherein one of said limited pluralnumber of different spectra illuminations is broad spectrum white light.15. A low cost broad spectrum spectrophotometer comprising means forsequentially illuminating a color test area with a limited plural numberof different spectra illuminations, and means for sequentially measuringthe reflected illumination from said sequentially illuminated color testarea by applying said reflected illumination simultaneously to multiplephoto-sites of a multi-photo-site photodetector, which simultaneouslyexposed multiple photo-sites comprise at least three different sets ofphoto-sites with at least three different illumination responsivespectral responses and three different parallel illumination responsivesignal outputs thereof.
 16. The low cost broad spectrumspectrophotometer of claim 15, wherein said limited plural number ofdifferent spectra illuminations is provided by three to four differentLEDs providing a corresponding limited number of different spectralilluminations, and a sequential actuation circuit for said LEDs.
 17. Thelow cost broad spectrum spectrophotometer of claim 15, wherein saidmulti-photo-site photodetector is a low cost commercial photodetectorchip which is normally a component part of a document color imaging bar,having at least three rows of small closely spaced photo-sites withintegral red, green and blue color filters respectively to provide saidat least three different spectral responses with at least threedifferent electrical output signals thereof in parallel.
 18. The lowcost broad spectrum spectrophotometer of claim 15, further includingcolor test area displacement insensitive optics means.
 19. A low costspectrophotometer comprising a broad spectrum white light illuminatorfor illuminating a color test target area and at least onemulti-photo-site photodetector, wherein said multi-photo-sitephotodetector is a low cost commercial photodetector chip which isnormally a component part of a document color imaging bar having atleast three rows of small closely spaced photo-sites with respectivered, green and blue color filters to provide at least three differentspectral responses of at least three different electrical outputsignals, said multi-photo-site photodetector being optically positionedto receive reflected light from said color test target area illuminatedby said broad spectrum white light illuminator.
 20. The low costspectrophotometer of claim 19, further including plural differentspectra LED illuminators and a sequential LED actuating circuit.
 21. Thelow cost spectrophotometer of claim 15, further including a lens systemand wherein said photodetector chip is oriented substantially in theplane of the image of said reflected light through said lens system.